Researchers photographed 10 men at four stages of beard growth: clean shaven, 5-day "light" stubble, 10-day "heavy" stubble (shown), and fully bearded. Three hundred and fifty-one women and 177 heterosexual men viewed the photos and rated each face for attractiveness, masculinity, health, and parenting ability.

Women ranked heavily stubbled faces as the most attractive. Participants said that the clean-shaven men looked about as healthy and attractive as those with a full beard, but rated the bearded men higher for perceived parenting skills. Light stubble got the short end of the stick, garnering low scores across the board from both men and women

The study was published in the May issue of the journal Evolution and Human Behavior. Here is a sample picture of one of the participants. The researchers said that a light beard may be too patchy to really get the manly effect through, while a full beard gives the impression of macho aggressiveness:Dixson and Brooks, Evo & Hum Behav, 2013An example of the stimuli used in this study. Images show the same man when clean-shaven, with light stubble, heavy stubble and a full beard.